The purpose of this project is to study the interaction between the animal host and tumors of tumor-associated antigens. The specific area investigated during the past year was the interaction of the major envelope glycoprotein (gp70) of Rauscher murine leukemia virus (RLV) with specific receptors on different mouse lymphoid cell populations (thymus, bone marrow, spleen). The results show that these lymphoid cell populations have different numbers of binding sites for RLV gp70 and suggest that viral gp70s from different MuLVs compete differentially for RLV gp70 receptors on different cells.